Hydrazine-free solution deposition of chalcogenide films

ABSTRACT

A method of depositing a film of a metal chalcogenide including the steps of: contacting an isolated hydrazinium-based precursor of a metal chalcogenide and a solvent having therein a solubilizing additive to form a solution of a complex thereof; applying the solution of the complex onto a substrate to produce a coating of the solution on the substrate; removing the solvent from the coating to produce a film of the complex on the substrate; and thereafter annealing the film of the complex to produce a metal chalcogenide film on the substrate. Also provided is a process for preparing an isolated hydrazinium-based precursor of a metal chalcogenide as well as a thin-film field-effect transistor device using the metal chalcogenides as the channel layer.

This application is a Divisional of U.S. application Ser. No. 11/506,827filed Aug. 21, 2006 which is a Divisional of U.S. application Ser. No.10/801,766 filed Mar. 16, 2004, now U.S. Pat. No. 7,094,651 which is aContinuation-in-part of U.S. application Ser. No. 10/617,118, filed Jul.10, 2003, now U.S. Pat. No. 6,875,661.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods of depositing a film of a metalchalcogenide from hydrazine-free solutions and a method of preparing afield-effect transistor including a film of a metal chalcogenidedeposited from hydrazine-free solutions. More particularly, the presentinvention relates to a method of preparing a field-effect transistorincluding as the channel layer a film of a metal chalcogenide depositedfrom hydrazine-free solutions.

2. Description of the Prior Art

The ability to deposit high quality semiconducting, metallic andinsulating thin films forms one of the important pillars of present-daysolid-state electronics. A solar cell may include, for example, a thinn-type semiconductor layer (˜0.25 μm) deposited on a p-type substrate,with electrical contacts attached to each layer to collect thephotocurrent. Light-emitting diodes (LED's) are typically comprised of ap-n bilayer, which under proper forward bias conditions emits light.

Thin-film field-effect transistors, denoted here as TFT's, include thinp- or n-type semiconducting channel layers, in which the conductivity ismodulated by application of a bias voltage to a conducting gate layerthat is separated from the channel by a thin insulating barrier. Theelectronic materials that include modern semiconducting devices havetypically been silicon based, but can equally be considered from otherfamilies of materials, in some cases potentially offering advantagesover the silicon-based technologies.

Thin-film field-effect transistors (TFT's), are widely used as switchingelements in electronic applications, most notably for logic and drivercircuitry within processor and display applications. Presently, TFT'sfor many lower-end applications, including those employed in activematrix liquid crystal displays, are made using amorphous silicon as thesemiconductor. Amorphous silicon provides a cheaper alternative tocrystalline silicon—a necessary condition for reducing the cost oftransistors for large area applications. Application of amorphoussilicon is limited, however, to slower speed devices, since the mobility(˜10⁻¹ cm²/V-sec) is approximately 15,000 times smaller than that ofcrystalline silicon.

In addition, although amorphous silicon is cheaper to deposit thancrystalline silicon, deposition of amorphous silicon still requirescostly processes such as plasma enhanced chemical vapor deposition. Thesearch for alternative semiconductors (i.e., not silicon), for use inTFT's and other electronic devices is therefore being vigorouslypursued.

If a semiconducting material could be identified which simultaneouslyprovides higher mobility and low-cost processing at moderate/lowtemperatures, many new applications can be envisioned for thesematerials, including light, flexible, very large-area displays orelectronics constructed entirely on plastic.

Recently, organic semiconductors have received considerable attention aspotential replacements for inorganic counterparts in TFT's (see, forexample, U.S. Pat. No. 5,347,144 assigned to Garnier et. al., entitled“Thin-Layer Field Effect Transistor With MIS Structure Whose Insulatorand Semiconductor Are Made of Organic Materials”) and LED's [S. E.Shaheen et al., “Organic Light-Emitting Diode with 20 Im/W EfficiencyUsing a Triphenyldiamine Side-Group Polymer as the Hole TransportLayer,” Appl. Phys. Lett. 74, 3212 (1999)].

Organic materials have the advantage of simple and low-temperaturethin-film processing through inexpensive techniques such as spincoating, ink jet printing, thermal evaporation, or stamping. Over thelast few years, the carrier mobilities of the organic channel layers inOTFTs (organic TFTs) have increased dramatically from <10⁻⁴ to ˜1cm²/V-sec (comparable to amorphous silicon) [see, for example, C. D.Dimitrakopoulos and D. J. Mascaro, “Organic thin-film transistors: Areview of recent advances,” IBM J. Res. & Dev. 45, 11-27 (2001)].

While very promising with regard to processing, cost, and weightconsiderations, organic compounds generally have a number ofdisadvantages, including poor thermal and mechanical stability. Inaddition, while the electrical transport in organic materials hasimproved substantially over the last 15 years; the mobility isfundamentally limited by the weak van der Waals interactions betweenorganic molecules (as opposed to the stronger covalent and ionic forcesfound in extended inorganic systems).

The inherent upper bound on electrical mobility translates to a cap onswitching speeds and therefore on the types of applications that mightemploy the low-cost organic devices. Organic semiconductors aretherefore primarily being considered for lower-end applications.

One approach to improving mobility/durability involves combining theprocessibility of organic materials with the desirable electricaltransport and thermal/mechanical properties of inorganic semiconductorswithin hybrid systems [D. B. Mitzi et al., “Organic-inorganicElectronics,” IBM J. Res. & Dev. 45, 29-45 (2001)]. Organic-inorganichybrid films have recently been employed as the semiconductive elementin electronic devices, including TFTs (see, for example, U.S. Pat. No.6,180,956, assigned to Chondroudis et al., entitled “Thin-FilmTransistors with Organic-Inorganic Hybrid Materials as SemiconductingChannels”) and LEDs (see, for example, U.S. Pat. No. 6,420,056, assignedto Chondroudis et.al., entitled “Electroluminescent Device WithDye-Containing Organic-Inorganic Hybrid Materials as an EmittingLayer”).

Several simple techniques have been described for depositing crystallineorganic-inorganic hybrid films, including multiple-source thermalevaporation, single source thermal ablation, and melt processing.

Solution-deposition techniques (e.g., spin coating, stamping, printing)have also received recent attention and are particularly attractivesince they enable the quick and inexpensive deposition of the hybrids ona diverse array of substrates. TFT's based on a spin-coatedsemiconducting tin(II)iodide-based hybrid have yielded mobilities ashigh as 1 cm²/V-sec (similar to the best organic-based devices preparedusing vapor-phase deposition and amorphous silicon).

Melt-processing of the hybrid systems has improved the grain structureof the semiconducting films, thereby leading to higher mobilities of 2-3cm²/V-sec [D. B. Mitzi et.al., “Hybrid Field-Effect Transistors Based ona Low-Temperature Melt-Processed Channel Layer,” Adv. Mater. 14,1772-1776 (2002)].

While very promising, current examples of hybrid semiconductors arebased on an extended metal halide frameworks (e.g., metal chlorides,metal bromides, metal iodides, most commonly tin(II) iodide). Metalhalides are relatively ionic in nature, thereby limiting the selectionof possible semiconducting systems with potential for high mobility. Inaddition, the tin(II)-iodide-based systems in particular are highly airsensitive and all processing must by done under inert-atmosphereconditions. Furthermore, while the tin(II)-iodide-based systems arep-type semiconductors, it is also desirable to find examples of n-typesystems, to enable applications facilitated by complementary logic. Sofar none have been identified.

Another alternative to silicon-based, organic, and metal-halide-basedhybrid semiconductors involves the use of metal chalcogenides (e.g.,metal sulfides, metal selenides, metal tellurides) as semiconductiveelements for use within TFT's and other electronic devices. Some of theearliest solar cells [D. C. Raynolds et al. “Photovoltaic Effect inCadmium Sulfide,” Phy. Rev. 96, 533 (1954)] and TFTs [P. K. Weimer, “TheTFT—A New Thin-Film Transistor,” Proc. IRE 50, 1462-1469 (1964)] were infact based on metal chalcogenide active layers. There are numerousexamples of metal chalcogenide systems that are potentially useful assemiconductive materials. Tin(IV) sulfide, SnS₂, is one candidate thathas generated substantial interest as a semiconducting material forsolar cells, with n-type conductivity, an optical band gap of ˜2.1 eVand a reported mobility of 18 cm²/V-sec [G. Domingo et al., “FundamentalOptical Absorption in SnS₂ and SnSe₂,” Phys. Rev. 143, 536-541 (1966)].

These systems might be expected to yield higher mobility than theorganic and metal-halide-based hybrids, as a result of the more covalentnature of the chalcogenides, and also provide additional opportunitiesfor identifying n-type semiconductors.

Reported mobilities of metal chalcogenides, for example, include SnSe₂(27 cm²/V-sec/n-type) [G. Domingo et al., “Fundamental OpticalAbsorption in SnS₂ and SnSe₂,” Phys. Rev. 143, 536-541 (1966)], SnS₂ (18cm²/V-sec/n-type) [T. Shibata et al., “Electrical Characterization of2H—SnS₂ Single Crystals Synthesized by the Low Temperature ChemicalVapor Transport Method,” J. Phys. Chem. Solids 52, 551-553 (1991)], CdS(340 cm²/V-sec/n-type), CdSe (800 cm²/V-sec/n-type) [S. M. Sze, “Physicsof Semiconductor Devices,” John Wiley & Sons, New York, 1981, p. 849],ZnSe (600 cm²/V-sec/n-type), and ZnTe (100 cm²/V-sec/p-type) [B. G.Streetman, “Solid State Electronic Devices,” Prentice-Hall, Inc., NewJersey, 1980, p. 443].

While the potential for higher mobility exists, the increased covalencyof the extended metal chalcogenide systems also reduces their solubilityand increases the melting temperature, rendering simple and low-costthin-film deposition techniques for these systems a significantchallenge.

A number of techniques have been proposed and employed for thedeposition of chalcogenide-based films, including thermal evaporation[A. Van Calster et.al., “Polycrystalline cadmium selenide films for thinfilm transistors,” J. Crystal Growth 86, 924-928 (1988)], chemical vapordeposition (CVD) [L. S. Price et al., “Atmospheric Pressure CVD of SnSand SnS₂ on Glass,” Adv. Mater. 10, 222-225 (1998)], galvanic deposition[B. E. McCandless et al., “Galvanic Deposition of Cadmium Sulfide ThinFilms,” Solar Energy Materials and Solar Cells 36, 369-379 (1995)],chemical bath deposition [F. Y. Gan et al., “Preparation of Thin-FilmTransistors With Chemical Bath Deposited CdSe and CdS Thin Films,” IEEETransactions on Electron Devices 49, 15-18 (2002)], and successive ioniclayer adsorption and reaction (SILAR) [B. R. Sankapal et al.,“Successive ionic layer adsorption and reaction (SILAR) method for thedeposition of large area (˜10 cm²) tin disulfide (SnS₂) thin films,”Mater. Res. Bull. 35, 2027-2035 (2001)].

However, these techniques are generally not amenable to low-cost,high-thoughput (fast) solution-based deposition techniques such asspin-coating, printing and stamping. What is required for application ofthe high-thoughput techniques is a truly soluble precursor for thechalcogenide and a suitable solvent. Given the potential for highelectrical mobility in the metal chalcogenide systems and both n- andp-channel devices, if a convenient and rapid solution-based techniquecould be identified for their deposition, the field of low-costsolution-based electronics, currently primarily being pursued within thecontext of organic electronics, might be extended to higher-endapplications, such as logic circuitry and very large area displays.

Spray pyrolysis is one technique employing the rapid decomposition of asoluble precursor [M. Krunks et al., “Composition of CuInS₂ thin filmsprepared by spray pyrolysis,” Thin Solid Films 403-404, 71-75 (2002)].The technique involves spraying a solution, which contains the chloridesalts of the metal along with a source of the chalcogen (e.g.,SC(NH₂)₂), onto a heated substrate (generally in the range 250-450° C.).

While metal chalcogenide films are formed using this technique, thefilms generally have substantial impurities of halogen, carbon ornitrogen. Annealing in reducing atmospheres of H₂ or H₂S at temperaturesup to 450° C. can be used to reduce the level of impurities in the film,but these relatively aggressive treatments are not compatible with awide range of substrate materials and/or require specialized equipment.

Ridley et al. [B. A. Ridley et al., “All-inorganic Field EffectTransistors Frabricated by Printing,” Science 286, 746-749 (1999)]describes CdSe semiconducting films that are printed using a solublemetal chalcogenide precursor formed using organic derivatized CdSenanocrystals. This technique, however, requires the formation ofnanocrystals with tight control on particle size distribution in orderto enable effective sintering during a postdeposition thermal treatment.The particle size control requires repeated dissolution andcentrifugation steps in order to isolate a suitably uniform collectionof nanocrystals.

Further, reported TFT devices prepared using this technique exhibitedunusual features, including substantial device hysteresis and a negativeresistance in the saturation regime, perhaps as a result of trap orinterface states either within the semiconducting film or at theinterface between the semiconductor and the insulator.

Dhingra et al. have also demonstrated a soluble precursor for metalchalcogenides that can be used to spin coat films of the correspondingmetal chalcogenide after thermal treatment to decompose the precursor(see S. Dhingra et al., “The use of soluble metal-polyselenide complexesas precursors to binary and ternary solid metal selenides,” Mat. Res.Soc. Symp. Proc. 180, 825-830 (1990).

However, in this process, the species used to solublize the chalcogenideframework (i.e., quarternary ammonium or phosphonium polyselenides),which ultimately decompose from the sample during the heat treatment,are very bulky and most of the film disappears during the annealingsequence (e.g., 70-87%). The resulting films consequently exhibitinferior connectivity and quality.

The large percentage of the sample that is lost during the thermaltreatment implies that only relatively thick films can be depositedusing this technique, since thin films would be rendered discontinuous(the above mentioned study considered films with thickness ˜25-35 μm).Additionally, relatively high temperatures are required for the thermaldecomposition of the polyselenides (˜530° C.), making this processincompatible with even the most thermally robust plastic substrates(e.g., Kapton sheet can withstand temperatures as high as 400° C.).

A study has also concluded that films of crystalline MoS₂ can be spincoated from a solution of (NH₄)₂MoS₄ in an organic diamine [J. Pütz andM. A. Aegerter, “Spin-Coating of MoS₂ Thin Films,” Proc. ofInternational Congress on Glass, vol. 18, San Francisco, Calif., Jul.5-10, 1998, 1675-1680]. However, high=temperature post-depositionanneals are required to achieve crystalline films (600-800° C.),rendering the process incompatible with organic-based flexible substratematerials.

A similar procedure has led to the formation of amorphous As₂S₃ andAs₂Se₃ films [G. C. Chern and I. Lauks, “Spin-Coated AmorphousChalcogenide Films,” J. Applied Phys. 53, 6979-6982 (1982)], butattempts to deposit other main-group metal chalcogenides, such as Sb₂S₃and GeS_(x) have not been successful, due to the low solubility of theprecursors in the diamine solvents [J: Pütz and M. A. Aegerter,“Spin-Coating of MoS₂ Thin Films,” Proc. of International Congress onGlass, vol. 18, San Francisco, Calif., Jul. 5-10, 1998, 1675-1680].

Improvement in the above-described solution-based processes aretherefore required for practical applications, especially for thefabrication of crystalline films of main-group metal chalcogenides, suchas those derived from Ge, Sn, Pb, Sb, Bi, Ga, In, Tl, which canpotentially provide high mobility in a semiconductor with reasonableband gap for a transistor.

If semiconducting materials and processing techniques could beidentified which simultaneously provide high carrier mobility andlow-cost processing at moderate/low temperatures, many new applicationscould be envisioned for these technologies, including light, flexible,very large-area displays or electronics constructed entirely on plastic.

The commonly owned co-pending U.S. application Ser. No. 10/617,118,Filed on Jul. 10, 2003, entitled “Solution Deposition of ChalcogenideFilms”, the contents of which are incorporated herein by reference,describes a novel solution-deposition process for preparingsemiconducting chalcogenide films by a process that employs a metalchalcogenide in a hydrazine compound to deposit the chalcogenide films.

While this process enables the deposition of high quality ultrathinspin-coated films with field-effect mobilities as high as 10 cm²/V-sec,approximately an order of magnitude higher than previous examples ofspin-coated semiconductors, the process has the disadvantage that itrequires spinning the film from a solution containing hydrazine, whichis a highly toxic and reactive substance.

In view of this disadvantage, it is highly desirable to provide aprocess, which avoids the use of hydrazine as the solvent altogether inthe process of depositing high quality ultrathin spin-coated films.

Accordingly, it is highly desirable to provide a process for preparationof semiconducting chalcogenide films without the use of hydrazine as thesolvent to overcome the high toxicity and reactivity concerns and,simultaneously, reduce the environmental impact and manufacturing costof the fabricated films, without the need for additional specializedequipment which otherwise would be required to handle a toxic solventsuch as hydrazine.

SUMMARY OF THE INVENTION

The present invention provides a hydrazinium metal chalcogeniderepresented by the formula:M_(z)X_(q)(R¹R²N—NHR³R⁴)_(2q-nz)(R¹R²N—NR³R⁴)_(m)wherein: M is a main-group metal having a valence n, wherein n is aninteger from 1 to 6; X is a chalcogen; z is an integer from 1 to 10; qis an integer from 1 to 30; m is from 1 to 30.5; and each of R¹, R², R³and R⁴ is independently selected from hydrogen, aryl, methyl, ethyl anda linear, branched or cyclic alkyl of 3-6 carbon atoms.

The present invention also provides a method of depositing a film of ametal chalcogenide. The method includes the steps of: contacting anisolated hydrazinium-based precursor of a metal chalcogenide and asolvent having therein a solubilizing additive to form a solution of acomplex thereof; applying the solution of the complex onto a substrateto produce a coating of the solution on the substrate; removing thesolvent from the coating to produce a film of the complex on thesubstrate; and thereafter annealing the film of the complex to decomposethe complex and produce a metal chalcogenide film on the substrate.

The present invention further provides a method of forming afield-effect transistor of the type having a source region and a drainregion, a channel layer extending between the source region and thedrain region, the channel layer including a semiconducting material, agate region disposed in spaced adjacency to the channel layer, anelectrically insulating layer between the gate region and the sourceregion, drain region and channel layer, including:

preparing a channel layer including a film of a metal chalcogenidesemiconducting material which is by a method including the steps of:contacting an isolated hydrazinium-based precursor of a metalchalcogenide and a solvent having therein a solubilizing additive toform a solution of a complex thereof; applying the solution of thecomplex onto a substrate to produce a coating of the solution on thesubstrate; removing the solvent from the coating to produce a film ofthe complex on the substrate; and thereafter annealing the film of thecomplex to decompose the complex and produce a metal chalcogenide filmon the substrate.

The present invention still further provides a first process forpreparing an isolated hydrazinium-based precursor of a metalchalcogenide. The first process includes the steps of: contacting: atleast one metal chalcogenide, a hydrazine compound represented by theformula:R¹R²N—NR³R⁴wherein each of R¹, R², R³ and R⁴ is independently selected fromhydrogen, aryl, methyl, ethyl and a linear, branched or cyclic alkyl of3-6 carbon atoms, and optionally an elemental chalcogen selected from S,Se, Te and a combination thereof, to produce a solution of ahydrazinium-based precursor of the metal chalcogenide in the hydrazinecompound; and isolating the hydrazinium-based precursor of the metalchalcogenide as a substantially pure product.

The present invention additionally provides a second process forpreparing an isolated hydrazinium-based precursor of a metalchalcogenide. The second process includes the steps of: contacting: atleast one metal chalcogenide and a salt of an amine compound with H₂S,H₂Se or H₂Te, wherein the amine compound is represented by the formula:NR⁵R⁶R⁷wherein each of R⁵, R⁶ and R⁷ is independently selected from hydrogen,aryl, methyl, ethyl and a linear, branched or cyclic alkyl of 3-6 carbonatoms, to produce an ammonium-based precursor of the metal chalcogenide;contacting the ammonium-based precursor of the metal chalcogenide, ahydrazine compound represented by the formula:R¹R²N—NR³R⁴wherein each of R¹, R², R³ and R⁴ is independently selected fromhydrogen, aryl, methyl, ethyl and a linear, branched or cyclic alkyl of3-6 carbon atoms, and optionally, an elemental chalcogen selected fromS, Se, Te and a combination thereof; to produce a solution of ahydrazinium-based precursor of the metal chalcogenide in the hydrazinecompound; and isolating hydrazinium-based precursor of the metalchalcogenide as a substantially pure product.

A hydrazinium metal chalcogenide according to the present invention canbe prepared as an isolated product in a substantially pure form byeither the first or second process described above.

The films prepared according to the present invention have the advantageof being low cost. The devices that employ metal chalcogenidesemiconducting materials as the channel layer yield the highestmobilities currently reported for an n-type spin-coated device.

Additionally, the device characteristics are better behaved than thosereported by Ridley et al. al. [B. A. Ridley et al., “All-Inorganic FieldEffect Transistors Fabricated by Printing,” Science 286, 746-749(1999)], in which CdSe semiconducting films are printed using a solublemetal chalcogenide precursor formed using organic derivatized CdSenanocrystals.

An important advantage of the present invention results from the factthat the present invention separates the steps of forming the solublehydrazinium based precursor from the solution processing the thin filmso that a chemical company could prepare the hydrazinium precursor andthe device maker could spin the films. Thus, a chemical company canprepare the hydrazinium precursor more efficiently because it isequipped to handle the toxic and reactive hydrazine and hydrazinederivatives in the plant. The device maker could then buy the precursorand spin the films according to its needs by using a less toxic solventin its manufacturing plants.

The low-cost metal chalcogenide semiconducting materials producedaccording to the present invention can be used in a variety ofapplications, including flat panel displays, non-linearoptical/photo-conductive devices, chemical sensors, emitting and chargetransporting layers in light-emitting diodes, thin-film transistors,channel layers in field-effect transistors and media for optical datastorage, including phase change media for optical data storage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the Thermogravimetric Analysis (TGA) scan of thehydrazinium tin(IV) sulfide precursor.

FIG. 2 depicts the X-ray diffraction pattern of a tin(IV) sulfide filmdeposited by Method 1 using spin coating.

FIG. 3 depicts the X-ray crystal structure of (N₂H₄)₃(N₂H₅)₄Sn₂Se₆,including Sn₂Se₆ ⁴⁻ dimers alternating with hydrazinium cations andneutral hydrazine molecules.

FIG. 4 depicts the X-ray diffraction patterns of tin(IV) selenideprecursor films deposited by Method 1 using spin coating.

FIG. 5 depicts the Thermogravimetric Analysis (TGA) scan of the ammoniumtin(IV) sulfide precursor, (NH4)_(x)SnS_(y).

FIG. 6 depicts the X-ray diffraction patterns of tin(IV) sulfideprecursor films deposited by Method 2 using spin coating.

FIG. 7 depicts X-ray diffraction patterns of antimony(III) sulfideprecursor films deposited by Method 2 using spin coating.

FIG. 8 is a Schematic diagram of a TFT device structure employing aspin-coated metal chalcogenide semiconductor as the channel material.

FIG. 9 depicts plots of I_(D) and I_(D) ^(1/2) versus V_(G) at constantV_(D)=100 V for a TFT with a spin-coated SnS₂ channel fabricated usingMethod 1.

FIG. 10 depicts plots of Drain current, I_(D), versus source-drainvoltage, V_(D), as a function of the gate voltage, V_(G), for a TFT witha spin-coated SnS₂ channel fabricated using Method 1.

FIG. 11 depicts plots of I_(D) and I_(D) ^(1/2) versus V_(G) at constantV_(D)=100 V for a TFT with a spin-coated SnS₂ channel fabricated usingMethod 2.

FIG. 12 depicts plots of drain current, I_(D), versus source-drainvoltage, V_(D), as a function of the gate voltage, V_(G), for a TFT witha spin-coated SnS₂ channel fabricated using Method 2.

FIG. 13 depicts plots of I_(D) and I_(D) ^(1/2) versus V_(G) at constantV_(D)=100 V, for a TFT with a spin-coated SnSe_(2-x)S_(x) channelfabricated using Method 2.

FIG. 14 depicts plots of drain current, I_(D) versus source-drainvoltage, V_(D) as a function of the gate voltage, V_(G), for a TFT witha spin-coated SnSe_(2-x)S_(x) channel fabricated using Method 2.

FIG. 15 depicts the crystal structure of the hydrazinium precursor,(N₂H₅)₄Sn₂S₆, determined using single crystal X-ray diffraction.

FIG. 16 shows the device characteristics for spin-coated SnS₂ channelwith a channel length L=95 μm and channel width W=1500 μm fabricatedusing Method 3. Drain current, I_(D), versus source-drain voltage,V_(D), as a function of gate voltage V_(G).

FIG. 17 shows the device characteristics for spin-coated SnS_(2.)channel with a channel length L=95 μm and channel width W=1500 μmfabricated using Method 3. Plots of I_(D) and I_(D) ^(1/2) versus V_(G)at constant V_(D)=85V, used to calculate current modulation,I_(ON)/I_(OFF), and saturation regime mobility, μ_(sat), for the tinsulfide channel.

FIG. 18 shows device characteristics for spin-coated SnS_(2-x)Se_(x)channel with a channel length L=95 μm and channel width W=1000 μmfabricated using Method 3. Drain current, I_(D), versus source-drainvoltage, V_(D), as a function of gate voltage, V_(G).

FIG. 19 shows device characteristics for spin-coated SnS_(2-x)Se_(x)channel with a channel length L=95 μm and channel width W=1000 μmfabricated using Method 3.

FIG. 20 shows X-ray photoemission spectroscopy (XPS) results. The Snvalency appears to change with the metal/chalocogenide ratio. As theS/Sn ratio is varied from 1.55 to 2.02, the Sn3d core level bindingenergy shifts from 486.3 eV to 487.2 eV corresponding to a decrease inSn(II) valency and an increase in Sn(IV) valency. A film grown withinsufficient S content has metallic rather than semiconductingcharacteristics.

DETAILED DESCRIPTION OF THE INVENTION

The previously incorporated commonly owned co-pending U.S. applicationSer. No. 10/617,118, Filed on Jul. 10, 2003, entitled “SolutionDeposition of Chalcogenide Films” describes novel solution-depositionmethods (Method 1 and Method 2) for preparing semiconductingchalcogenide films by a process that employs a metal chalcogenide in ahydrazine compound to deposit the chalcogenide films. Thehydrazinium-based precursor in these methods is formed in situ, i.e.,the hydrazine compound, which is toxic and hazardous to handle, is thesolvent. Thus, in Method 1 and Method 2, the hydrazinium-based precursoris not isolated.

Formation of Films from Isolated Hydrazinium-based Precursor

In contrast to the approach in Methods 1 and 2, the present inventionprovides a new method, namely Method 3, of depositing thin films ofmetal chalcogenides using an isolated hydrazinium-based precursor of themetal chalcogenide to deposit the chalcogenide films. This method avoidsthe use of the hydrazine compound in the method of depositing thechalcogenide films.

The method of depositing the chalcogenide films in which an isolatedhydrazinium-based precursor is used, i.e., Method 3, includes the stepsof:

contacting an isolated hydrazinium-based precursor of a metalchalcogenide, or mixtures of metal chalcogenides, and a solvent havingtherein a solubilizing additive to form a solution of a complex thereof;

applying the solution of the complex onto a substrate to produce acoating of the solution on the substrate;

removing the solvent from the coating to produce a film of the complexon the substrate; and thereafter

annealing the film of the complex to decompose the complex and produce ametal chalcogenide film on the substrate.

The step of contacting an isolated hydrazinium-based precursor of ametal chalcogenide and a solvent that has therein a solubilizingadditive produces a complex of the metal chalcogenide in which thehydrazine has been substantially replaced by the solubilizing additiveto form the complex.

Suitable solvents include water, lower alcohols, such as, methanol,ethanol, propanol, iso-propanol, n-butanol, iso-butyl alcohol, sec-butylalcohol, cyclohexanol, ethers, such as, diethyl ether, dibutyl ether,esters, such as, ethyl acetate, propyl acetate, butyl acetate, othersolvents, such as, alkylene glycol of 2-6 carbon atoms, dialkyleneglycol of 4-6 carbon atoms, trialkylene glycol of 6 carbon atoms, glyme,diglyme, triglyme, propylene glycol monoacetate, DMSO, DMF, DMA, HMPAand a mixture thereof.

The solubilizing additive can be an aliphatic amine of 1-10 carbonatoms, aromatic amine of 4-10 carbon atoms, aminoalcohol of 2-6 carbonatoms or a mixture thereof.

Preferably, the aliphatic amine is selected from n-propylamine,iso-propylamine, n-butylamine, sec-butylamine, iso-butylamine,pentylamine, n-hexylamine, cyclohexylamine and phenethylamine, and thearomatic amine preferably is pyridine, aniline or aminotoluene. Theaminoalcohol is preferably selected from ethanolamine, propanolamine,diethanolamine, dipropanolamine, triethanolamine and tripropanolamine.Other suitable amines can also be used provided they promote solubilityof the hydrazinium-based precursor of a metal chalcogenide in thesolvent selected.

The film deposition of the hydrazinium-based precursor is carried out bystandard solution-based techniques using a solution of an isolatedhydrazinium-based precursor in a solvent. Suitable solution-basedtechniques include spin coating, stamping, printing; or dip coating,using the above-mentioned solution. After removing the solvent, a shortlow-temperature anneal (typically, at a temperature less than about 350°C.) is carried out to decompose the hydrazinium chalcogenide salts fromthe sample and to improve the crystallinity of the resulting metalchalcogenide film.

The present invention provides semiconducting materials and processingtechniques which simultaneously provide high carrier mobility andlow-cost processing at moderate/low temperatures, with many newapplications of these technologies, including light, flexible, verylarge-area displays or electronics constructed entirely on plastic.

The present invention enables the deposition of high quality ultrathinspin-coated films with field-effect mobilities as high as 10 cm²/V-sec,approximately an order of magnitude higher than previous examples ofspin-coated semiconductors.

The present invention includes four stages:

1. Isolating (synthesizing) the hydrazinium precursor of the metalchalcogenide compound;

2. Forming a solution of the hydrazinium precursor in a suitably chosennon-hydrazine-based solvent mixture;

3. Solution processing a thin film of the chalcogenide precursor fromthe solution mixture described in stage 2 using a technique such as spincoating, stamping or printing; and

4. Decomposing the resulting precursor film into a film of the desiredmetal chalcogenide using heat achieved, for example, by placing the filmon a hot plate, in an oven, or by using laser-based annealing.

Preferably, solution processing in stage 3 is accomplished by spincoating.

The heating of the film in stage 4 is preferably accomplished by placinga substrate containing the film on a hot plate with the temperature setbetween 50° C. and 450° C. for a period of time between 0.5 min to 120min.

The metal chalcogenide compound described in stage 1 preferably includesa main group metal and a chalcogen (e.g., SnS₂, SnSe₂, SnS_(2-x)Se_(x),In₂Se₃, GeSe₂, Sb₂S₃, Sb₂Se₃, Sb₂Te₃).

The non-hydrazine-based solvent of stage 2 preferably includes anorganic amine (e.g., propylamine, butylamine, phenethylamine,etylenediamine).

The non-hydrazine-based solvent mixture of stage 2 preferably includesexcess chalcogen (S, Se, Te) mixed with the organic amine to control theresulting metal chalcogenide stoichiometry and improve the filmformation.

Preferably, the metal chalcogenide film is in the form of a thin filmand the thin film has a thickness of from about 5 Å to about 2,000 Å,more preferably from about 5 Å to about 1,000 Å.

The metal chalcogenide film can include a polycrystalline metalchalcogenide which has a grain size equal to or greater than thedimensions between contacts in a semiconductor device. However, themetal chalcogenide film can include single crystals of the metalchalcogenide.

Annealing is carried out at a temperature and for a length of timesufficient to produce the metal chalcogenide film. Preferably, thetemperature is from about 25° C. to about 500° C. More preferably, thetemperature is from about 250° C. to about 350° C.

Using this technique, tertiary or higher order systems, such as,(SnS_(2-x)Se_(x)) can also be conveniently formed as thin films,enabling more detailed control over the band gap of the materialsdeposited. Thus, the present invention can be used most advantageouslyto form main-group metal (e.g., Ge, Sn, Pb, Sb, Bi, Ga, In, Tl)chalcogenide thin films.

As a first demonstration of the present invention, films of SnS₂ aredeposited and characterized. In this case, the metal chalcogenide filmscan be deposited using spin coating, although they could equally bedeposited using other solution-based techniques.

Following film deposition, low-temperature heat treatment of the film,typically at less than about 350° C., yields a crystalline film of thedesired metal chalcogenide, with the loss of hydrazine and hydraziniumchalcogenide and the decomposition products of these compounds.

The films prepared by any of the methods of the present invention can beremoved from the substrate to produce an isolated film thereof.

Preferably, the substrate is fabricated from a material having at leastone property selected from the following: thermally stable, i.e., stableup to about at least 300° C., chemically inert towards the metalchalcogenides, rigid or flexible. Suitable examples include Kapton,polycarbonate, silicon, amorphous hydrogenated silicon, silicon carbide(SiC), silicon dioxide (SiO₂), quartz, sapphire, glass, metal,diamond-like carbon, hydrogenated diamond-like carbon, gallium nitride,gallium arsenide, germanium, silicon-germanium, indium tin oxide, boroncarbide, boron nitride, silicon nitride (Si₃N₄), alumina (Al₂O₃),cerium(IV) oxide (CeO₂), tin oxide (SnO₂), zinc titanate (ZnTiO₂), aplastic material or a combination thereof. Preferably, the metalsubstrate is a metal foil, such as, aluminum foil, tin foil, stainlesssteel foil and gold foil, and the plastic material preferably ispolycarbonate, Mylar or Kevlar.

The processes described herein are useful in forming semiconductor filmsfor applications including, for example, thin-film transistors (TFTs),light-emitting devices (LED's) and data storage media.

There are numerous examples of metal chalcogenide systems that arepotentially useful as semiconductive materials. Tin(IV) sulfide, SnS₂,is one candidate that has generated substantial interest as asemiconducting material for solar cells, with n-type conductivity, anoptical band gap of ˜2.1 eV and a reported mobility of 18 cm²/V-sec [G.Domingo et al., “Fundamental Optical Absorption in SnS₂ and SnSe₂,”Phys. Rev. 143, 536-541 (1966)].

Preparation of Isolated Hydrazinium-Based Precursor

The present invention further provides a first and a second process forpreparing the hydrazinium-based precursor of a metal chalcogenide as anisolated product.

The first process for preparing the hydrazinium-based precursor includesthe steps of:

contacting at least one metal chalcogenide, a hydrazine compoundrepresented by the formula:R¹R²N—NR³R⁴wherein each of R¹, R², R³ and R⁴ is independently selected fromhydrogen, aryl, methyl, ethyl and a linear, branched or cyclic alkyl of3-6 carbon atoms, and optionally an elemental chalcogen selected from S,Se, Te and a combination thereof, to produce a solution of ahydrazinium-based precursor of the metal chalcogenide in the hydrazinecompound; and

isolating the hydrazinium-based precursor of the metal chalcogenide as asubstantially pure product.

A possible mechanism that may be operative in the above system is thefollowing:N₂H₄+2X→N₂ (gas)+2H₂X  (1)4N₂H₄+2H₂X+2MX₂→4N₂H₅₊+M₂X₆ ⁴⁻(in solution)  (2)

wherein M=a metal and X═S or Se.

The second process for preparing the hydrazinium-based precursorincludes the steps of:

contacting: at least one metal chalcogenide and a salt of an aminecompound with H₂S, H₂Se or H₂Te, wherein the amine compound isrepresented by the formula:NR⁵R⁶R⁷wherein each of R⁵, R⁶ and R⁷ is independently selected from hydrogen,aryl, methyl, ethyl and a linear, branched or cyclic alkyl of 3-6 carbonatoms, to produce an ammonium-based precursor of the metal chalcogenide;

contacting the ammonium-based precursor of the metal chalcogenide, ahydrazine compound represented by the formula:R¹R²N—NR³R⁴wherein each of R¹, R², R³ and R⁴ is independently selected fromhydrogen, aryl, methyl, ethyl and a linear, branched or cyclic alkyl of3-6 carbon atoms, and optionally, an elemental chalcogen selected fromS, Se, Te and a combination thereof; to produce a solution of ahydrazinium-based precursor of the metal chalcogenide in the hydrazinecompound; and

isolating hydrazinium-based precursor of the metal chalcogenide as asubstantially pure product.

Preferably, each of R⁵, R⁶ and R⁷ can independently be hydrogen, aryl,methyl and ethyl. More preferably, R⁵, R⁶ and R⁷ are all hydrogens.

Preferably, in this process each of R¹, R², R³ and R⁴ is independentlyhydrogen, aryl, methyl or ethyl. More preferably, the hydrazine compoundis hydrazine, i.e., where R¹, R², R³ and R⁴ are all hydrogens,methylhydrazine or 1,1-dimethylhydrazine.

Preferably, the amine compound is NH₃, CH₃NH₂, CH₃CH₂NH₂, CH₃CH₂CH₂NH₂,(CH₃)₂CHNH₂, CH₃CH₂CH₂CH₂NH₂,phenethylamine, 2-fluorophenethylamine,2-chlorophenethylamine, 2-bromophenethylamine, 3-fluorophenethylamine,3-chlorophenethylamine, 3-bromophenethylamine, 4-fluorophenethylamine,4-chlorophenethylamine, 4-bromophenethylamine,2,3,4,5,6-pentafluorophenethylamine or a combination thereof.

The ammonium metal chalcogenide precursor can be prepared by a varietyof techniques depending on the metal chalcogenide under consideration.Examples of such techniques include simple dissolution of the metalchalcogenide in an ammonium chalcogenide aqueous solution followed byevaporation of the solution, typically at room temperature, solvothermaltechniques and by solid-state routes at elevated temperatures. Incontrast to most metal chalcogenides, which are not substantiallysoluble in common solvents, the ammonium salts can be highly soluble inhydrazine with the vigorous evolution of ammonia and the formation ofthe hydrazinium salts of the metal chalcogenide [L. F. Audrieth et al.,“The Chemistry of Hydrazine,” John Wiley & Sons, New York, 200 (1951)].For example, the solubility of the ammonium-based SnS2 precursor,exceeds 200 g/l in hydrazine.

Isolation of the hydrazinium-based precursor of the metal chalcogenideas a substantially pure product from the first or second process of thepresent invention can be accomplished by any suitable method orprocedure known to a person skilled in the art. Such suitable methodsinclude, but are not limited to, the following:

1. evaporation of the solvent to produce the hydrazinium-based precursorof the metal chalcogenide as a substantially pure solid;

2. precipitation of the hydrazinium-based precursor of the metalchalcogenide by addition of a solvent in which the hydrazinium-basedprecursor of the metal chalcogenide is less soluble or substantiallyinsoluble followed by filtration of the precipitate; and

3. cooling of the reaction mixture to precipitate the hydrazinium-basedprecursor of the metal chalcogenide from the reaction mixture followedby filtration of the precipitate.

Because the hydrazine moieties that solubilize the metal chalcogenidestructure only weakly interact with the metal chalcogenide framework,they can be conveniently removed from the precursor film at lowtemperatures. Additionally, the starting materials are all chalcogenides(not halides or oxides), and therefore impurities of these elements areabsent from the final films.

The present invention is distinct from the earlier disclosed use ofhydrazine hydrate as a solvent for the precipitation of certain metalsulfides and selenides (e.g., zinc sulfide, copper selenide,silver-doped zinc sulfide, copper-doped zinc cadmium sulfide) [U.S. Pat.No. 6,379,585, assigned to Vecht et al., entitled “Preparation ofSulfides and Selenides]”. In the case of this previous work, the solvent(which always involves water, as well as hydrazine) generally enablesthe precipitation of a transition metal chalcogenide, rather than thedissolution of the metal chalcogenide for further solution-basedprocessing.

The present techniques are not limited to the use of hydrazine. Otherhydrazine-like solvents such as 1,1-dimethyl-hydrazine andmethylhydrazine or mixtures of hydrazine-like solvents with othersolvents, including water, methanol, ethanol, acetonitrile andN,N-dimethylformamide, can also be used.

The chalcogenide can further include an elemental chalcogen, such as, S,Se, Te or a combination thereof.

The hydrazinium-based precursor, i.e., hydrazinium metal chalcogenide,is represented by the formula:M_(z)X_(q)(R¹R²N—NHR³R⁴)_(2q-nz)(R¹R²N—NR³R⁴)_(m)

wherein:

M is a main-group metal having a valence n, wherein n is an integer from1 to 6;

X is a chalcogen;

z is an integer from 1 to 10;

q is an integer from 1 to 30;

m is from 1 to 30.5; and

each of R¹, R², R³ and R⁴ is independently selected from hydrogen, aryl,methyl, ethyl and a linear, branched or cyclic alkyl of 3-6 carbonatoms.

The metal chalcogenides include a metal, such as, Ge, Sn, Pb, Sb, Bi,Ga, In, Tl or a combination thereof and a chalcogen, such as, S, Se, Teor a combination thereof.

In one embodiment, the metal chalcogenide can be represented by theformula MX or MX₂ wherein M is a metal, such as, Ge, Sn, Pb or acombination thereof and wherein X is a chalcogen, such as, S, Se, Te ora combination thereof.

In another embodiment, the metal chalcogenide can be represented by theformula M₂X₃ wherein M is a metal, such as, Sb, Bi, Ga, In or acombination thereof and wherein X is a chalcogen, such as, S, Se, Te ora combination thereof.

In yet another embodiment, the metal chalcogenide can be represented bythe formula M₂X wherein M is Tl and wherein X is a chalcogen, such as,S, Se, Te or a combination thereof.

Preferably, the metal is Sn or Sb and the chalcogen is S or Se.

Examples of such chalcogenide systems include compounds represented bythe formula:Sn(S_(2-x)Se_(x))wherein x is from 0 to 2, including SnS₂ and SnSe₂.Thin Film Field Effect Transistor Devices

The present invention further provides an improved thin-filmfield-effect transistor (TFT) device with a semiconducting channel layerthat is deposited using the above-described 4 stage approach.

Such devices prepared according to the present invention yield thehighest mobilities currently reported for an n-type solution-processeddevice. Additionally, the device characteristics are better behaved thanthose reported by Ridley et al. [B. A. Ridley et al., “All-InorganicField Effect Transistors Fabricated by Printing,” Science 286, 746-749(1999)], in which CdSe semiconducting films are printed using a solublemetal chalcogenide precursor formed using organic derivatized CdSenanocrystals.

Accordingly, the present invention includes an improved field-effecttransistor, based on a solution-processed chalcogenide channel layerusing a method that employs an isolated hydrazinium based precursoraccording to the present invention. The field-effect transistor is athin film field-effect transistor (FET) device having a film of a metalchalcogenide semiconducting material as the active semiconductor layer.

The present invention provides a method of preparing a field-effecttransistor of the type having a source region and a drain region, achannel layer extending between the source region and the drain region,the channel layer including a semiconducting material, a gate regiondisposed in spaced adjacency to the channel layer, an electricallyinsulating layer between the gate region and the source region, drainregion and channel layer, including:

preparing a channel layer including a film of a metal chalcogenidesemiconducting material by a method including the steps of: contactingan isolated hydrazinium-based precursor of a metal chalcogenide and asolvent having therein a solubilizing additive to form a solution of acomplex thereof; applying the solution of the complex onto a substrateto produce a coating of the solution on the substrate; removing thesolvent from the coating to produce a film of the complex on thesubstrate; and thereafter annealing the film of the complex to decomposethe complex and produce a metal chalcogenide film on the substrate.

In one embodiment, the source region, channel layer and drain region arepreferably disposed upon a surface of a substrate, the electricallyinsulating layer is disposed over the channel layer and extending fromthe source region to the drain region, and the gate region is disposedover the electrically insulating; layer, for example, as shown in FIG. 4of U.S. Pat. No. 6,180,956, the contents of which are incorporatedherein by reference.

In another embodiment, the gate region is disposed as a gate layer upona surface of a substrate, the electrically insulating layer is disposedupon the gate layer, and the source region, channel layer, and drainregion are disposed upon the electrically insulating layer, for example,as shown in FIG. 3 of the previously incorporated U.S. Pat. No.6,180,956.

Preferably, the metal chalcogenide semiconducting material is in theform of a thin film, in which the metal chalcogenide semiconductingmaterial is a polycrystalline material having a grain size equal to orgreater than the dimensions between contacts in the semiconductordevice. Accordingly, the present invention can provide an improvedfield-effect transistor prepared by the aforementioned method.

Formation of Films without Isolation of the Hydrazinium-based Precursor

The present invention further provides a simple method (Method 1) ofdepositing thin films of metal chalcogenides involving the use of ahydrazine (or other hydrazine-like solvents)/chalcogenide mixture assolvent for metal chalcogenides or mixtures of metal chalcogenides. Inthis method, the hydrazinium-based precursor is not isolated. The filmdeposition of the hydrazinium-based precursor is carried out by standardsolution-based techniques including spin coating, stamping, printing, ordip coating, using the above-mentioned solution. Thereafter, a shortlow-temperature anneal (typically, at a temperature less than about 350°C.) is carried out to remove excess hydrazine and hydraziniumchalcogenide salts from the sample and to improve the crystallinity ofthe resulting metal chalcogenide film.

The first method, in which the hydrazinium-based precursor is notisolated, includes the steps of:

contacting: at least one metal chalcogenide; a hydrazine compoundrepresented by the formula:R¹R²N—NR³R⁴wherein each of R¹, R², R³ and R⁴ is independently hydrogen, aryl,methyl, ethyl or a linear, branched or cyclic alkyl of 3-6 carbon atoms;and optionally, an elemental chalcogen, such as, S, Se, Te or acombination thereof; to produce a solution of a hydrazinium-basedprecursor of the metal chalcogenide;

applying the solution of the hydrazinium-based precursor of the metalchalcogenide onto a substrate to produce a film of the precursor; andthereafter

annealing the film of the precursor to remove excess hydrazine andhydrazinium chalcogenide salts to produce a metal chalcogenide film onthe substrate.

The present invention also provides a simple method (Method 2) ofdepositing thin films of metal chalcogenides involving: (1) thesynthesis of a soluble ammonium-based precursor; (2) the use ofhydrazine as a solvent for the precursor; (3) the deposition of a filmusing a standard solution-based technique (mentioned above) and (4)low-temperature annealing. The annealing step can be carried out at atemperature from about room temperature to about 500° C., but typicallyit is carried out at a temperature from about 250° C. to about 350° C.).

The second method (Method 2) in which the hydrazinium-based precursor isnot isolated, is similar to the first method (Method 1), except that achalcogenide and an amine are first contacted to produce anammonium-based precursor of the metal chalcogenide, which is thencontacted with a hydrazine compound and and optionally, an elementalchalcogen. This method includes the steps of:

contacting: at least one metal chalcogenide and a salt of an aminecompound with H₂S, H₂Se or H₂Te, wherein the amine compound isrepresented by the formula:NR⁵R⁶R⁷wherein each of R⁵, R⁶ and R⁷ is independently hydrogen, aryl, methyl,ethyl or a linear, branched or cyclic alkyl of 3-6 carbon atoms, toproduce an ammonium-based precursor of the metal chalcogenide;

contacting the ammonium-based precursor of the metal chalcogenide, ahydrazine compound represented by the formula:R¹R²N—NR³R⁴wherein each of R¹, R², R³ and R⁴ is independently hydrogen, aryl,methyl, ethyl or a linear, branched or cyclic alkyl of 3-6 carbon atoms,and optionally, an elemental chalcogen, such as, S, Se, Te or acombination thereof, to produce a solution of a hydrazinium-basedprecursor of the metal chalcogenide in the hydrazine compound;

applying the solution of the isolated hydrazinium-based precursor of themetal chalcogenide onto a substrate to produce a film of the precursor;and thereafter

annealing the film of the precursor to remove excess hydrazine andhydrazinium chalcogenide salts to produce a metal chalcogenide film onthe substrate.

Referring to the Figures:

FIG. 1 depicts the Thermogravimetric Analysis (TGA) scan of thehydrazinium tin(IV) sulfide precursor, synthesized from SnS₂, S, andhydrazine, and run at 2° C./min to 800° C. in flowing nitrogen.

FIG. 2 depicts the X-ray diffraction pattern of a tin(IV) sulfide filmdeposited by Method 1 using spin coating and annealed at 300° C. for 10min. The calculated c-axis parameter is 5.98 Å, consistent with thepublished bulk sample values for SnS₂, 5.90 Å [B. Palosz et al., J.Appl. Crystallogr. 22, 622 (1989)].

FIG. 3 depicts the X-ray crystal structure of (N₂H₄)₃(N₂H₅)₄Sn₂Se₆,including Sn₂Se₆ ⁴⁻ dimers alternating with hydrazinium cations andneutral hydrazine molecules.

FIG. 4 depicts the X-ray diffraction patterns of tin(IV) selenideprecursor films deposited by Method 1 using spin coating and annealed at(a) 225° C., (b) 250° C., (c) 275° C., (d) 300° C. The films exhibitincreasing crystallinity with increasing anneal temperature, as well assubstantial c-axis preferred orientation. The calculated c-axisparameter using the 300° C. film is 6.13 Å, consistent with publishedbulk sample values for SnSe₂.

FIG. 5 depicts the Thermogravimetric Analysis (TGA) scan of the ammoniumtin(IV) sulfide precursor, (NH4)_(x)SnS_(y), run at 2° C./min to 800° C.in flowing nitrogen.

FIG. 6 depicts the X-ray diffraction patterns of tin(IV) sulfideprecursor films deposited by Method 2 using spin coating and annealed at(a) unannealed, (b)140° C., (c) 200° C., (d) 250° C., (e) 325° C., (f)400° C. The films exhibit increasing crystallinity with increasinganneal temperature, as well as substantial c-axis preferred orientation.The calculated c-axis parameter using the 400° C. film is 5.95 Å,consistent with published bulk sample values for SnS₂, 5.90 Å [B. Paloszet al., J. Appl. Crystallogr. 22, 622 (1989)].

FIG. 7 depicts the X-ray diffraction patterns of antimony(III) sulfideprecursor films deposited by Method 2 using spin coating and annealed at(a) 250° C. and (b) 325° C. The reflection indices, based on a publishedstructure report for Sb₂S₃, are given in the figure [D. Nodland et al.,North Dakota State University, Fargo, N.Dak., USA, ICDD Grant-in-Aid(1990)].

FIG. 8 is a Schematic diagram of a TFT device structure employing aspin-coated metal chalcogenide semiconductor as the channel material.

FIG. 9 depicts the Plots of I_(D) and I_(D) ^(1/2) versus V_(G) atconstant V_(D)=100 V, used to calculate current modulation,I_(on)/I_(off), and saturation-regime field-effect mobility, μ, for aTFT with a spin-coated SnS₂ channel of length L=25 μm and width W=1500μm, fabricated using Method 1. The gate dielectric is 3000 Å SiO₂.

FIG. 10 depicts plots of Drain current, I_(D), versus source-drainvoltage, V_(D), as a function of the gate voltage, V_(G), for a TFT witha spin-coated SnS₂ channel of length L=25 μm and width W=1500 μm,fabricated using Method 1. The gate dielectric is 3000 Å SiO₂.

FIG. 11 depicts plots of I_(D) and I^(D) ^(1/2) versus V_(G) at constantV_(D)=100 V, used to calculate current modulation, I_(on)/I_(off), andsaturation-regime field-effect mobility, μ, for a TFT with a spin-coatedSnS₂ channel of length L=25 μm and width W=1500 μm, fabricated usingMethod 2. The gate dielectric is 3000 Å SiO₂.

FIG. 12 depicts plots of drain current, I_(D), versus source-drainvoltage, V_(D), as a function of the gate voltage, V_(G), for a TFT witha spin-coated SnS₂ channel of length L=25 μm and width W=1500 μm,fabricated using Method 2. The gate dielectric is 3000 Å SiO₂.

FIG. 13 depicts plots of I_(D) and I_(D) ^(1/2) versus V_(G) at constantV_(D)=100 V, used to calculate current modulation, I_(on)/I_(off), andsaturation-regime field-effect mobility, μ, for a TFT with a spin-coatedSnSe_(2-x)S_(x) channel of length L=25 μm and width W=1500 μm,fabricated using Method 2. The gate dielectric is 3000 Å SiO₂.

FIG. 14 depicts plots of drain current, I_(D), versus source-drainvoltage; V_(D), as a function of the gate voltage, V_(G), for a TFT witha spin-coated SnSe_(2-x)S_(x) channel of length L=25 μm and width W=1500μm, fabricated using Method 2. The gate dielectric is 3000 Å SiO₂.

FIG. 15 depicts the crystal structure of the hydrazinium precursor,(N₂H₅)₄Sn₂S₆, determined using single crystal X-ray diffraction.Hydrogen atoms have been removed for clarity. Unit cell (outlined bydashed lines): Orthorhombic (P 2₁ 2₁ 2₁), a=8.5220(5) A, b=13.6991 (8)A, c=14.4102 (9) A, V=1682.301A³, and Z=4.

FIG. 16 shows the device characteristics for spin-coated SnS₂ channelwith a channel length L=95 μm and channel width W=1500 μm fabricatedusing Method 3. The gate dielectric is 2500 Å SiO₂. Drain current,I_(D), versus source-drain voltage, V_(D), as a function of gate voltageV_(G).

FIG. 17 shows the device characteristics for spin-coated SnS₂ channelwith a channel length L=95 μm and channel width W=1500 μm fabricatedusing Method 3. The gate dielectric is 2500 Å SiO₂. Plots of I_(D) andI_(D) ^(1/2) versus V_(G) at constant V_(D)=85V, used to calculatecurrent modulation, I_(ON)/I_(OFF), and saturation regime mobility,μ_(sat), for the tin sulfide channel.

FIG. 18 shows device characteristics for spin-coated SnS_(2-x)Se_(x)channel with a channel length L=95 μm and channel width W=1000 μmfabricated using Method 3. The gate dielectric is 2500 Å SiO₂. Draincurrent, I_(D), versus source-drain voltage, V_(D), as a function ofgate voltage, V_(G).

FIG. 19 shows device characteristics for spin-coated SnS_(2-x)Se_(x)channel with a channel length L=95 μm and channel width W=1000 μmfabricated using Method 3. The gate dielectric is 2500 Å SiO₂. Plots ofI_(D) and I_(D) ^(1/2) versus V_(G) at constant V_(D)=85V, used tocalculate current modulation, I_(ON)/I_(OFF), and saturation regimemobility, μ_(sat), for the SnS_(2-x)Se_(x) channel.

FIG. 20 shows the X-ray photoemission spectroscopy (XPS) results. The Snvalency appears to change with the metal/chalocogenide ratio. As theS/Sn ratio is varied from 1.55 to 2.02, the Sn3d core level bindingenergy shifts from 486.3 eV to 487.2 eV, corresponding to a decrease inSn(II) valency and an increase in Sn(IV) valency. A film grown withinsufficient S content has metallic rather than semiconductingcharacteristics.

It is important to obtain the correct metal/chalcogenide ratio in theprecursor solution. Insufficient quantities of chalocogenide will renderthe film excessively conductive. For example, a film grown withinsufficient S content has more metallic rather than semiconductingcharacteristics.

This can be seen in the Sn core level binding energy measured with x-rayphotoemission, which is an indicator of the valency of the Sn. If weexamine the Sn3d spectra of a film processed with a conventionalhydrizinium precursor, i.e., with hydrazine as the solvent, we see abroad peak centered at 486.6 eV binding energy (FIG. 20).

A film grown using the present invention without extra S has a corelevel binding energy of 486.3 eV, which moves to 487.2 eV if additionalS is used. The shift in binding energy indicates a trend from Sn(II)valency towards Sn(IV) valency, with the more highly ionized species ata higher binding energy. We conclude that the addition of S has reducedthe quantity of SnS and increased the SnS2 content of the film.

MEIS determination of the S/Sn ratio for these films also indicate atrend towards improved stoichiometry that correlates with the XPSresults. The films labeled “no N2H4” are fabricated using the methods ofthe present invention.

Film S/Sn No N2H2 (sn10) 1.55 N2H2 (sn08) 1.8 No N2H2 extra S (sn11)2.02

The present invention provides a new method of solution-depositingchalcogenide-based electronic materials in the form of high-quality thinfilms for application in electronic devices. Ability to deposit fromsolution is particularly attractive since it opens the opportunity toemploy a number of low-cost; low-temperature, relatively quicktechniques such as spin-coating, printing, stamping, and dipping. Whilethe discussion presented below primarily refers to the application ofthese films in TFT's, this discussion is meant to be representative, andthe electronic films so deposited could equally be used in otherelectronic devices as well.

Illustrative examples of the preparation of films of metal chalcogenidesemiconducting materials and TFT devices thereof are provided below.

METHOD 1 Example 1

A solution of SnS₂ was formed by dissolving 0.274 g SnS₂ (1.5 mmol) in1.0 ml hydrazine and 0.048 g sulfur (1.5 mmol). Substantial bubbling isobserved during the dissolution process, presumably as a result ofevolution of primarily nitrogen during the reaction. The dissolution issomewhat slow at room temperature, requiring approximately 4-6 hours ofstirring to produce a clear yellow solution. The solution was filteredand the filtrate was evaporated under a flow of nitrogen gas, ultimatelyyielding 0.337 g of yellow powder. Thermal analysis of the powderindicates that the resulting solid decomposes to form SnS₂ at arelatively low temperature, essentially by 200° C., with a weight lossof 38% observed during the process (FIG. 1).

A film of the above-described precursor can readily be deposited on acleaned quartz substrate by dissolving 0.055 g SnS₂ (0.3 mmol) in 0.5 mlhydrazine and 0.010 g sulfur (0.3 mmol). Each substrate was precleanedsequentially in aqua regia, toluene, methanol and ultimately in aconcentrated aqueous ammonium hydroxide solution. Thin films of SnS₂ areformed by depositing 3-4 drops of the above-mentioned SnS₂ solution ontothe substrate, allowing the substrate/solution to sit for 20 min toimprove the wetting, and spinning the substrate at 2000 rpm for 2 min inair. The resulting yellow films are annealed 300° C. for 10 minutes inan inert atmosphere. X-ray patterns for the resulting film are shown inFIG. 2 and indicate a crystalline film of SnS₂.

Example 2

SnSe₂ films were also deposited using the above-described technique.

0.277 g SnSe₂ (1 mmol) was readily dissolved (in several minutes, withstirring) in 1 ml hydrazine when 0.079 g Se (1 mmol) have been added,yielding ultimately a light yellow solution. The light yellow colorationsuggests the lack of formation of polychalcogenides during the processalthough if larger quantities of Se are added, a darker coloration ofthe solution is observed. Substantial bubbling is observed during thedissolution process, presumably as a result of evolution of primarilynitrogen during the reaction. Evaporating the solution under flowingnitrogen gas over the period of several hours leads to the formation ofapproximately 0.450 g of a yellow crystalline powder. The powder rapidlyloses weight while sitting on a balance at room temperature, suggestingincorporation of solvent (i.e., hydrazine) within the structure, whichslowly dissociates from the sample.

Single crystal structure refinement on a crystal collected from theproduct (FIG. 3) has composition (N₂H₄)₃(N₂H₅)₄Sn₂Se₆ and a structureincluding Sn₂Se₆ ⁴⁻ dimers alternating with hydrazinium cations andneutral hydrazine molecules. Note that the structure of the productsupports the mechanism drawn out in equations (1) and (2).

Thin films of SnSe₂ are formed by depositing 3 drops of theabove-mentioned SnSe₂ solution onto a substrate (e.g., quartz), allowingthe substrate/solution to sit for 20 min to improve the wetting, andspinning the substrate at 2000 rpm for 2 min. The resulting yellow filmsare annealed at 225° C., 250° C., 275° C., and 300° C. for 10 minutes inan inert atmosphere. X-ray patterns for the resulting films are shown inFIG. 4, indicating increased crystallinity with increasing annealtemperature. However, even at the low temperatures (i.e., <<300° C.),crystalline films can be prepared. The X-ray diffraction pattern is inagreement with that for SnSe₂, and additionally indicates substantialpreferred orientation.

Films of SnSe_(2-x)S_(x) (x≈1) have similarly been prepared startingwith SnS₂, SnSe₂, S, Se and hydrazine, respectively.

METHOD 2

In accordance with the second method of the present invention for thepreparation of a metal chalcogenide solution, an ammonium metalchalcogenide precursor can be prepared, depending on the metalchalcogenide under consideration, by any suitable technique including asimple dissolution of the metal chalcogenide in an ammonium chalcogenideaqueous solution followed by evaporation of the solution at roomtemperature, solvothermal techniques and by solid-state routes atelevated temperatures.

Example 3

In this example, the ammonium-based tin(IV) sulfide precursor wassynthesized by dissolving 548.5 mg of SnS₂ (3 mmol) in 85 ml of 50 wt. %aqueous ammonium sulfide, (NH₄)₂S, over a period of 4 days. The solutionwas filtered through a 0.45 μm glass microfiber filter and the resultingfiltered solution was slowly evaporated under a flow of nitrogen gasover a period of several days; leading to a yellow product (˜1.05 g).

The product was repeatedly washed filtered with methanol until theresulting filtrate was colorless and the thermal analysis scan on thefinal product yields a weight-loss transition at ˜150° C. (to yieldSnS₂) of approximately 30% (FIG. 5). This indicates an approximatecomposition, (NH₄)₂SnS₃. The product is nominally amorphous, i.e., noX-ray pattern is observed in a powder diffractometer. The weight loss,which corresponds to loss of ammonia and hydrogen sulfide, is initiatedat temperatures as low as ambient temperature, indicating the ease withwhich the material can be decomposed to SnS₂. Because the SnS₂ precursorneed not have a fixed stoichiometric ratio, the precursor is referred toherein as (NH₄)_(x)SnS_(y).

The precursor, (NH₄)_(x)SnS_(y), was found to be highly soluble inhydrazine. For example, 80 mg of (NH₄)_(x)SnS_(y) can be easilydissolved in 0.5 ml anhydrous hydrazine, with vigorous evolution ofammonia. Accordingly, films can be conveniently spin-coated on quartzdisks from the above mentioned hydrazine solution, either in an inertatmosphere drybox or in air, using approximately 3-4 drops of theprecursor solution on each 0.75 inch quartz disk substrate and spinningat, for example, 2000 rpm for 2 minutes.

Each substrate was precleaned sequentially in aqua regia, toluene,methanol and ultimately in a concentrated aqueous ammonium hydroxidesolution. The resulting film was light yellow in color (transparent) andvery smooth in appearance.

Five annealing temperatures were considered: 140° C., 200° C., 250° C.,325° C., 400° C. For each temperature, the coated substrate was annealedat the prescribed temperature for 20 minutes on a temperature-controlledhot plate in an inert (nitrogen) atmosphere. X-ray studies of filmsindicate progressively improving crystallinity with increasing annealingtemperature (FIG. 6), although some crystallinity is noted attemperatures even as low as 200° C.

Example 4

As a second example of the ammonium-precursor technique, films of Sb₂S₃were prepared.

The ammonium-based antimony(III) sulfide precursor was synthesized bydissolving 0.250 g of Sb₂S₃ (0.74 mmol) in 10 ml of 50 wt. % aqueousammonium sulfide, (NH₄)₂S, over a period of several hours. The solutionwas filtered through a 0.45 μm glass microfiber filter and the resultingfiltered solution was slowly evaporated under a flow of nitrogen gasover a period of 3 hours, leading to a darkly-colored product (˜0.385g). The precursor, (NH₄)_(x)SbS_(y), was found to be highly soluble inhydrazine. For example, 80 mg of (NH₄)_(x)SnS_(y) can be easilydissolved in 0.5 ml anhydrous hydrazine, with vigorous evolution ofammonia. Films can therefore be conveniently spin-coated on quartz disksfrom the above mentioned-hydrazine solution, either in an inertatmosphere dry box or in air, using approximately 3-4 drops of theprecursor solution on each 0.75 inch quartz disk substrate and spinning,for example, at 2000 rpm for 2 minutes.

Each substrate was precleaned sequentially in aqua regia, toluene,methanol and ultimately in a concentrate ammonium hydroxide solution.The resulting film was lightly colored (transparent) and very smooth inappearance.

Two annealing temperatures were considered: 250° C. and 325° C. For eachtemperature, the coated substrate was annealed at the prescribedtemperature for 20 minutes on a temperature-controlled hot plate in aninert (nitrogen) atmosphere. Upon placing the substrate on the hotplate, the color of the film immediately darkened, indicating theformation of Sb₂S₃. X-ray study of films indicates progressivelyimproving crystallinity with increasing annealing temperature (FIG. 7).

Example 5

TFT Device:

TFT's were prepared, using the semiconducting metal chalcogenidesdescribed above as the semiconducting channel. For testing purposes, thedevices tested include a heavily n-doped silicon substrate (which alsoacts as the gate), 3000 Å thermally grown SiO₂ insulating barrier layer,a spin-coated tin(IV) sulfide or selenide channel layer, and patterned800 Å gold source and drain electrodes (FIG. 8).

For SnS₂ semiconducting channels, the spin-coating solution was preparedby either Method 1, i.e., dissolving 20 mg SnS₂ (0.11 mmol) in 1.6 mlhydrazine and 3.5 mg S (0.11 mmol), or Method 2, i.e., dissolving 25 mgof the ammonium precursor, (NH₄)_(x)SnS_(y), in 1.6 ml hydrazine. Ineither case, 3-4 drops of the tin(IV) sulfide solution are placed oncleaned 2 cm×2 cm silicon substrates (with the final cleaning stepincluded of placing the substrates in ammonium hydroxide for at least ½hr) and spun in air at 3500 rpm for 1 min. The annealing sequence forthe Method 1 film includes a gradual ramp to 120° C., a dwell at 120° C.for 20 min, and finally an anneal at 300° C. for approximately 5 min.The film received a second anneal at 300° C. after deposition of thegold source and drain contacts, which substantially improved devicecharacteristics.

The annealing sequence for the Method 2 film includes a gradual ramp to120° C., a dwell at 120° C. for 20 min, and finally an anneal at 300° C.for approximately 15 min. Drain and gate sweep characteristics for theSnS₂ channels produced using Method 1 or 2 are shown in FIGS. 9-12.

The device characteristics for the devices made using Method 1 areμ_(sat)=0.20 cm²/V-sec, μ_(lin)=0.10 cm²/V-sec, andI_(on)/I_(off)=7×10³, while for Method 2 the values are μ_(sat)=0.07cm²/V-sec, μ_(lin)=0.05 cm²/V-sec, and I_(on)/I_(off)=6×10³.

Example 6

Films of a mixed SnSe_(2-x)S_(x) type were also made using Method 2.

The ammonium precursor was synthesized by dissolving 0.360 g SnSe₂ (1.3mmol) in 56 ml of 50 wt. % aqueous ammonium sulfide, (NH₄)₂S, over aperiod of several days. The solution was filtered through a 0.45 μmglass microfiber filter and the resulting filtered solution was slowlyevaporated under a flow of nitrogen gas over a period of several days.The darkly-colored product was rinsed thoroughly with methanol, yielding0.477 g of a darkly-colored product. The tin(IV) selenide/sulfideprecursor layer was prepared by dissolving 30 mg of the precursor in 1.6ml hydrazine and spinning 3-4 drops placed on the substrate at 3500 rpmfor 1 min.

The oxide-coated silicon substrates were cleaned thoroughly and dippedin an ammonium hydroxide solution for at least an hour before spincoating of the metal chalcogenide solution. The annealing sequence forthe films includes a gradual ramp to 120° C., a dwell at 120° C. for 20min, and finally an anneal at 300° C. for approximately 15 min.

Device properties are shown in FIGS. 13 and 14, yielding at μ_(sat)=1.3cm²/V-sec, μ_(lin)=1.0 cm²/V-sec, and I_(on)/I_(off)=10³, some of thehighest reported values for mobility for an n-type spin-coatedsemiconductor.

METHOD 3

In the following examples, the hydrazinium precursors were isolated fromthe reaction mixture. Unlike Examples 1-6 in which the reaction mixturescontaining hydrazine were used “as is” to cast the films, in Examples7-9, the isolated precursors were used to cast the films.

Example 7

SnS₂ Films and Devices:

1. Pale yellow (N₂H₅)₄Sn₂S₆ crystals and powder are formed by dissolving(in a nitrogen atmosphere, over several hours, with stirring) 0.183 g ofSnS₂ (1 mmol) in 2 ml of hydrazine and 0.064g of S (2 mmol), yieldingultimately a yellow solution.

Note that hydrazine is highly toxic and should be handled usingappropriate protective equipment to prevent contact with either thevapors or liquid. Evaporating the solution to dryness under flowingnitrogen gas over the period of several hours, as well as under vacuumfor several hours, leads to the formation of approximately 0.274 g (0.49mmol) of the hydrazinium precursor [(N₂H₅)₄Sn₂S₆]. Chemical analysis ofthe product yields results consistent with expected values—observed:N(19.8%), H(3.5%); calculated: N(19.9%), H(3.6%). The crystal structureshown in FIG. 15 confirms the given chemical formula and indicates thatstructurally the precursor has anionic dimers of edge-sharing SnS₄tetrahedra (Sn₂S₆ ⁴⁻) alternating with hydrazinium cations.

2. A solution of the hydrazinium-based precursor, (N₂H₅)₄Sn₂S₆, Wasformed by stirring 14 mg of (N₂H₅)₄Sn₂S₆ and 3 mg S in 3.3 mlbutylamine, in a nitrogen atmosphere, for approximately 12 hours. Theresulting solution was clear yellow in appearance and suitable forsolution processing. Note that solutions have also been demonstratedusing phenethylamine and ethylenediamine as the solvent.

3. Solution processing of the thin film was accomplished by spincoating. The 2 cm×2 cm square silicon substrate was heavily n-doped,yielding a substrate resistivity <0.005 ohm-cm, and coated withapproximately 2500 angstroms of thermally grown SiO₂ (measuredcapacitance=1.30×10⁻⁸ μF/cm²). The substrate was cleaned using astandard process of a soap scrub with a cotton swab, sonication inethanol and dichloromethane, and a piranha process (hydrogenperoxide:sulfuric acid in 1:4 volume ratio). Approximately 12 drops ofthe above-described butylamine-based solution (stage 2), passed througha 0.2 mm filter, were placed on the cleaned silicon substrate. Thesolution was allowed to sit on the substrate for approximately 10 secbefore initiating the spinning cycle. The spinning cycle included a rampto 4000 rpm, 4000 rpm for 45 sec, and ramp to 0 rpm, yielding a film ofthe metal chalcogenide precursor. The film was further dried in air byplacing the the substrate (with film on top) on a preheated hot plate at120° C. for 5 minutes and then transferred into a nitrogen filled drybox for the final decomposition step.

4. The decomposition step was accomplished by placing the substrate(with the precursor film on top) on a preheated hot plate at 270° C. fora period of 20 minutes. The heating was performed in a nitrogenatmosphere with oxygen and water levels maintained below 1 ppm. Afterthe heat treatment, the film was removed from the hot plate and cooledto ambient temperature, yielding a semiconducting film of tin(IV)sulfide. The stoichiometry of films, as determined using medium energyion scattering (MEIS), was SnS_(2.0(1)), entirely consistent with theexpected SnS₂ composition. Note that films deposited from solutions inwhich no excess S was added (i.e., just hydrazinium precursor andbutylamine) generally yielded films which were sulfur deficient, i.e.,SnS_(2−x), where x is approximately 0.5. This resulted in films whichwere too conductive and therefore inappropriate for use in TFT channellayers (i.e., they were impossible to shut off).

TFT Device:

A thin film transistor was demonstrated by depositing gold source anddrain electrodes on top of the semiconducting film described in stage 4.The configuration of the device is shown in FIG. 8. A representativeplot of drain current, I_(D), versus drain voltage, V_(D), is shown inFIG. 16 as a S function of the applied gate voltage, V_(G), for a TFTwith a tin sulfide channel formed using the stages 1-4 described above.The device operates as an n-channel transistor, operating inaccumulation mode upon application of a positive gate bias. Applicationof a negative gate bias depletes the channel of electrons and shuts thedevice off. At low V_(D), the TFT shows typical transistor behavior asI_(D) increases linearly with V_(D). Current saturation, with a smallohmic component, is observed at high V_(D) as the accumulation layer ispinched off near the drain electrode. Current modulation(I_(ON)/I_(OFF)) and saturation regime field-effect mobility (μ_(sat))are calculated from the plot of I_(D) and I_(D) ^(1/2) versus V_(G)(FIG. 17), yielding I_(ON)/I_(OFF)=10⁴ and μ_(sat)=0.4 cm²/V-s,respectively, for a −60 to 85 V gate sweep and V_(D)=85 V. Note that useof a thinner or higher dielectric constant gate insulator (relative tothe 2500 Å SiO₂ layer currently used) is expected to enable significantreduction in the device operating voltage. The linear regime mobilityderived from the plots in FIG. 16, μ_(lin)=0.38 cm²/V-s, is very similarto the saturation regime value. Note that this is in contrast toorganic- and hybrid-based thin-film devices in which, generally, thelinear regime mobility value is substantially smaller than thesaturation regime value. As the discrepancy between these values isoften sited to arise from trap states, the absence of a discrepancy inthe current films likely atests to the high quality of the films.

Example 8

SnS_(2-x)Se_(x) Films and Devices:

1. Light brown (N₂H₅)₄Sn₂S_(6-x)Se_(x) crystals and powder are formed bydissolving (in a nitrogen atmosphere, over several hours, with stirring)0.292 g of SnS₂ (1.6 mmol) and 0.147 g of SnSe₂ (0.53 mmol) in 2 ml offreshly distilled hydrazine and 0.128 g of S (4 mmol), yieldingultimately a yellow-brown solution.

Note that hydrazine is highly toxic and should be handled usingappropriate protective equipment to prevent contact with either thevapors or liquid.

Evaporating the solution to dryness under flowing nitrogen gas over theperiod of several hours, as well as under vacuum for several hours,leads to the formation of approximately 0.628 g (0.49 mmol) of theproduct with expected approximate composition (N₂H₅)₄Sn₂S₅Se.

2. A solution of the hydrazinium-based precursor with approximatecomposition, (N₂H₅)₄Sn₂S₅Se, was formed by stirring 14 mg of theprecursor and 3 mg S in 3.3 ml butylamine, in a nitrogen atmosphere, forapproximately 2 hours. The resulting solution was clear brown inappearance and suitable for solution processing. Note that solutionshave also been demonstrated using phenethylamine and ethylenediamine asthe solvent.

3. Solution processing of the thin film was accomplished by spincoating. The 2 cm×2cm square silicon substrate was heavily n-doped,yielding a substrate resistivity <0.005 ohm-cm, and coated withapproximately 2500 angstroms of thermally grown SiO₂. The substrate wascleaned using a standard process of a soap scrub with a cotton swab,sonication in ethanol and dichloromethane, and a piranha process(hydrogen peroxide:sulfuric acid in 1:4 volume ratio). Approximately 12drops of the above-described butylamine-based solution (stage 2), passedthrough a 0.2 mm filter, were placed on the cleaned silicon substrate.The solution was allowed to sit on the substrate for approximately 10sec before initiating the spinning cycle. The spinning cycle included ofa ramp to 4000 rpm, 4000 rpm for 45 sec, and ramp to 0 rpm, yielding afilm of the metal chalcogenide precursor. The film was further dried inair by placing the the substrate (with film on top) on a preheatedhotplate at 120° C. for 5 minutes and then transferring the film into anitrogen filled dry box for the final decomposition step.

4. The decomposition step was accomplished by placing the substrate(with the precursor film on top) on a preheated hot plate at 250° C. fora period of 20 minutes. The heating was performed in a nitrogenatmosphere with oxygen and water levels maintained below 1 ppm. Afterthe heat treatment, the film was removed from the hot plate and cooledto ambient temperature, yielding a semiconducting film ofSnS_(2-x)Se_(x), with expected composition of approximatelySnS_(1.5)Se_(0.5).

TFT Device:

A thin film transistor was demonstrated by depositing gold source anddrain electrodes on top of the semiconducting film that was prepared asdescribed in stage 4. A representative plot of drain current, I_(D),versus drain voltage, V_(D), is shown in FIG. 18 as a function of theapplied gate voltage, V_(G), for a TFT with a SnS_(2-x)Se_(x) channelformed using the stages 1-4 described above. The device operates as ann-channel transistor, operating in accumulation mode upon application ofa positive gate bias. Application of a negative gate bias depletes thechannel of electrons and shuts the device off. At low V_(D), the TFTshows typical transistor behavior as I_(D) increases linearly withV_(D). Current saturation, with a small ohmic component, is observed athigh V_(D) as the accumulation layer is pinched off near the drainelectrode. Current modulation (I_(ON)/I_(OFF)) and saturation regimefield-effect mobility (μ_(sat)) are calculated from the plot of I_(D)and I_(D) ^(1/2) versus V_(G) (FIG. 19), yielding I_(ON)/I_(OFF)=10⁴ andμ_(sat)=1.1 cm²/V-s, respectively, for a −60 to 85 V gate sweep andV_(D)=85 V. Note that use of a thinner or higher dielectric constantgate insulator (relative to the 2500 Å SiO₂ layer currently used) isexpected to enable significant reduction in the device operatingvoltage. The linear regime mobility derived from the plots in FIG. 18,μ_(lin)=1.1 cm²/V-s, virtually the same as the saturation regime value.

Example 9

Hydrazinium precursors of the following main group metal chalcogenideshave also been isolated. They can be used, as in the examples above, toyield thin films suitable for device applications: SnSe₂, GeSe₂, In₂Se₃,Sb₂S₃, and Sb₂Se₃.

The described solution-processing technique is potentially valuablebecause of the ultrathin and relatively uniform nature of the resultingspin-coated films (i.e., ˜50 angstroms, or less than 10 unit cellsthick). This therefore opens the possibility of using the spin-coatingprocess to fabricate multilayered structures. For example, layers ofSnS₂ might alternate with layers of SnSe₂.

It might be advantageous to alternate large band gap semiconductors (orinsulators) with narrow band gap semiconductors to create quantum wellstructures or semiconductors with different band offset to create type 1or type 2 quantum well structures. By changing the concentration ofsolute in the solution or the spin speed, the relative thickness of thelayers of the structure can be controlled. The resulting multilayerstructures might be useful for device application in solar cells,thermoelectric or transistor devices.

The potential advantage of this process is that it would be much lowercost than the traditional MBE or vacuum evaporation approaches currentlyused for creating multilayer structures.

The present invention has been described with particular reference tothe preferred embodiments. It should be understood that variations andmodifications thereof can be devised by those skilled in the art withoutdeparting from the spirit and scope of the present invention.Accordingly, the present invention embraces all such alternatives,modifications and variations that fall within the scope of the appendedclaims.

1. A hydrazinium-metal chalgenide represented by the formula:M_(z)X_(q)(R¹R²N—NHR³R⁴)_(2q-nz)(R¹R²—NR³R⁴)_(m) wherein: M is amain-group metal having a valence n, wherein n is an integer from 1 to6; X is a chalcogen; z is an integer from 1 to 10; q is an integer from1 to 30; m is from 1 to 30.5; and each of R¹, R², R³ and R⁴ isindependently selected from the group consisting of: hydrogen, aryl,methyl, ethyl and a linear; branched or cyclic alkyl of 3-6 carbonatoms.
 2. The hydrazinium-metal chalgenide of claim 1, wherein saidmetal is selected from the group consisting of: Ge, Sn, Pb, Sb, Bi, Ga,In and Tl.
 3. The hydrazinium-metal chalgenide of claim 1, wherein saidchalcogenide is selected from the group consisting of: S, Se and Te. 4.The hydrazinium-metal chalgenide of claim 2, wherein said chalcogenideis selected from the group consisting of: S, Se and Te.
 5. Thehydrazinium-metal chalgenide of claim 1, wherein said metal chalcogenideis represented by the formula MX or MX₂ wherein M is a metal selectedfrom the group consisting of: Ge, Sn, Pb and a combination thereof; andwherein X is a chalcogen selected from the group consisting of S, Se, Teand a combination thereof.
 6. The hydrazinium-metal chalgenide of claim1, wherein said metal chalcogenide-is represented by the formula M₂X₃wherein M is a metal selected from the group consisting of: Sb, Bi, Ga,In and a combination thereof; and wherein X is a chalcogen selected fromthe group consisting of: S, Se, Te and a combination thereof.
 7. Thehydrazinium-metal chalgenide of claim 1, wherein said metal chalcogenideis represented by the formula M₂X wherein M is Tl; and wherein X is achalcogen selected from the group consisting of: S, Se, Te and acombination thereof.
 8. The hydrazinium-metal chalgenide of claim 1,wherein said metal is selected, from the group consisting of: Sn and Sb;and wherein said chalcogen is selected from the group consisting of: Sand Se.
 9. The hydrazinium-metal chalgenide of claim 1, wherein saidchalcogenide is represented by the formula: Sn(S_(2-x)Se_(x)) wherein xis from 0 to
 2. 10. The hydrazinium-metal chalgenide of claim 1, whereineach of R¹, R², R³ and R⁴ is independently selected from the groupconsisting of: hydrogen, aryl, methyl and ethyl.
 11. Thehydrazinium-metal chalgenide of claim 1, wherein R¹, R², R³ and R⁴arehydrogen.
 12. The hydrazinium-metal chalgenide of claim 2, wherein saidchalcogenide is represented by the formula: Sn(S_(2-x)Se_(x)) wherein xis from 0 to
 2. 13. The hydrazinium-metal chalgenide of claim 2, whereineach of R¹, R², R³ and R⁴ is independently selected from the groupconsisting of: hydrogen, aryl, methyl and ethyl.
 14. Thehydrazinium-metal chalgenide of claim 2, wherein R¹, R², R³ and R⁴ arehydrogen.
 15. The hydrazinium-metal chalgenide of claim 3, wherein eachof R¹, R², R³ and R⁴ is independently selected from the group consistingof: hydrogen, aryl, methyl and ethyl.
 16. The hydrazinium-metalchalgenide of claim 3, wherein R¹, R², R³ and R⁴are hydrogen.
 17. Ahydrazinium metal chalcogenide prepared by a process comprising:contacting: at least one metal chalcogenide and a salt of an aminecompound with H₂S, H₂Se or H₂Te, wherein said amine compound isrepresented by the formula:NR⁵R⁶R⁷ wherein each of R⁵, R⁶ and R⁷ is independently selected from thegroup consisting of: hydrogen, aryl, methyl, ethyl and a linear,branched or cyclic alkyl of 3-6 carbon atoms, to produce anammonium-based precursor of said metal chalcogenide; contacting saidammonium-based precursor of said metal chalcogenide, a hydrazinecompound represented by the formula:R¹R²N—NR³R⁴ wherein each of R¹, R², R³ and R⁴ is independently selectedfrom the group consisting of: hydrogen, aryl, methyl, ethyl and alinear, branched or cyclic alkyl of 3-6 carbon atoms, and optionally, anelemental chalcogen selected from the group consisting of: S, Se, Te anda combination thereof; to produce a solution of a hydrazinium-basedprecursor of said metal chalcogenide in said hydrazine compound; andisolating hydrazinium-based precursor of said metal chalcogenide as asubstantially pure product.
 18. The hydrazinium-metal chalgenide ofclaim 17, wherein each of R¹, R², R³ and R⁴ is independently selectedfrom the group consisting of hydrogen, aryl, methyl and ethyl.